This invention relates generally to balloon expandable stents and, in particular, to a flexible stent having a waveform pattern formed from a sheet of biocompatible material and into a cylindrical surface or tubular shape.
Vascular stents are deployed at a narrowed site in a blood vessel of a patient for widening the vessel lumen and circumferentially supporting the vessel wall. Vascular stents desirably present a small cross-sectional dimension or profile for introducing the stent into the affected vessel lumen.
One approach to providing a vascular stent is the use of a piece of wire bent into a number of turns. Although suitable for its intended use, a problem with these bent wire stents is that stress points are formed at each wire bend or turn. As a result, the wire stent is structurally compromised at a number of points. Furthermore, bent wire stents lack longitudinal stability. For example, a wire stent is typically positioned in a blood vessel over an inflatable balloon. The balloon expands first at opposite ends, where the balloon is not in contact with the wire stent. As a result, the wire stent is longitudinally compressed between the inflated balloon ends. With continued inflation, the middle of the balloon expands, thereby unevenly expanding the wire bends of the longitudinally compressed wire stent. In an attempt to remedy the problem, the stent wire material has been formed to cross over or attach to itself. A problem with this attempted remedy is that the cross-sectional dimension of the stent, or stent profile, is increased, and the stent intrudes into the effective lumen of the blood vessel. The effective lumen of the blood vessel is further constricted by the growth of endothelial tissue layers over the stent wire. As a result, the stent and tissue growth impede fluid flow and cause turbulence in the vessel lumen. Another problem with this attempted remedy is that galvanic action, exposure to a reactive surface, or ion migration, occurs at the wire-to-wire contact points. The wire stent material rubs when movement occurs during ordinary blood flow and pulsation as well as patient muscle movement.
Another approach to providing a vascular stent is the use of a piece of metal cannula with a number of openings formed in the circumference thereof. A problem with the use of a metal cannula stent is that the stent is rigid and inflexible. As a result, the stent is difficult, if not impossible, to introduce through the tortuous vessels of the vascular system for deployment at a narrowed site. Furthermore, the stent is too rigid to conform with a curvature of a blood vessel when deployed at an occlusion site. Another problem with the use of a metal cannula stent is that the stent longitudinally shrinks during radial expansion. As a result, the position of the metal cannula stent shifts, and the stent supports a shorter portion of the blood vessel wall than anticipated merely by stent length.
Yet another approach to providing a vascular stent is the use of a wire mesh that is rolled into a generally tubular shape. A problem with the use of a wire mesh stent is that the overlapping wires forming the mesh increase the stent profile, thereby reducing the effective lumen of the blood vessel. The growth of endothelial tissue layers over the wire mesh further reduces the effective blood vessel lumen. Another problem with this approach is that ion migration also occurs at the wire-to-wire contact points.
Still yet another approach to providing a vascular stent is the use of a flat metal sheet with a number of openings formed in rows therein. The flat metal sheet stent also includes three rows of fingers or projections positioned on one edge of the stent along the axis thereof. When expanded, a row of the fingers or projections is positioned through a row of openings on the opposite edge is of the stent for locking the expanded configuration of the stent. A problem with the use of the flat metal sheet stent is that the overlapping edges of the stent increase the stent profile. Again, the stent profile and endothelial growth reduce the effective blood vessel lumen. Another problem with the use of the flat metal sheet stent is that the fingers or projections along one edge of the stent make wire-to-wire contact with the opposite edge of the stent. As a result, the metal edges of the stent rub during movement caused by blood flow, pulsation, and muscle movement. Yet another problem with the use of the flat metal sheet stent is that the fingers or projections extend radially outwardly and into the vessel wall. As a result, the intimal layer of the vessel wall is scraped, punctured, or otherwise injured. Injury and trauma to the intimal layer of the vessel wall result in hyperplasia and cell proliferation, which in turn effect stenosis or further narrowing of the vessel at the stent site.
The foregoing problems are solved and a technical advance is achieved in an illustrative embodiment of a flexible stent comprising a waveform pattern that is formed from a sheet of malleable, biocompatible material having a specified uniform thickness. The pattern is formed into a tubular shape and into an overlapping state around a delivery catheter balloon for introduction through tortuous vessels to, for example, an occlusion site in a coronary vessel. To provide longitudinal flexibility while preventing longitudinal contraction or expansion of the stent during radial expansion of the stent, the pattern advantageously includes a reinforcing member extending longitudinally therealong. A plurality of cells extends laterally from the reinforcing member with selected of the closed cells each having a fixedly sized aperture therein. The closed cells are interposed when the stent is in the tubular shape. To minimize the thickness of the stent and the growth of endothelial cells therearound, each segment of the cells extends laterally from the reinforcing member and does not overlap itself or any adjacent laterally extending segment of the cells. The sheet of biocompatible material with the pattern formed therein is formed into a radially alterable tubular shape around a delivery catheter balloon for introduction to the occlusion site. The balloon radially expands the stent to engage the vessel wall surface and to maintain the vessel lumen in an open condition. The expanded stent in a nonoverlapping state advantageously has a minimal thickness for endothelial tissue to form thereover. As a result, the vessel lumen is advantageously maintained with the largest diameter possible.
The pattern of the stent when in the tubular shape includes an overlapping state in which at least one segment of the selected cells overlaps the reinforcing member and forms a combined thickness with and along the reinforcing member of no more than substantially twice the thickness of the sheet of material. A deflated, delivery catheter balloon is positioned within the tubular-shaped stent to radially expand the stent to a nonoverlapping, expanded state when positioned at the occlusion site. The outermost longitudinal edges of the tubular stent move radially and circumferentially relative to each other when the stent is being radially altered. These outermost edges advantageously engage the surface of the lumen wall to maintain the stent in the expanded state. These outermost edges are most evident on the curved end segments of the interposed cells of the pattern when in the tubular shape. To aid expansion of the stent with the delivery balloon, the stent surface material is treated to lower its coefficient of friction. In one instance, the treatment comprises a coating of parylene on the surface of the sheet of material. Other coating materials include polytetrafluoroethylene. Furthermore, the surface of the stent may be ion beam bombarded to advantageously change the surface energy density and the coefficient of friction.
To maintain the moment of inertia or stiffness of the stent, each segment of the cells has a width substantially greater than the specified thickness of the sheet material. Increasing the width of the laterally extending segments also increases the surface area of the stent and support of the vessel wall.
To increase the expansion ratio of the stent, the laterally extending cells may be formed around the reinforcing member more than once and within the aperture of a closed cell without each segment overlapping itself or any adjacent cell segment. The width of the cell along the reinforcing member is advantageously selected so that each laterally extending segment forms a predetermined angle so as not to overlap itself or any adjacent cell segment. This is to advantageously maintain a combined thickness with and along the reinforcing member of no more than substantially twice the thickness of the sheet of material.
Radiopaque markers are advantageously positioned at one or more ends of the waveform pattern to aid the physician in positioning the stent at the occlusion site.
The method of making the balloon expandable stent includes the steps of providing a sheet of malleable material having an initial surface area and removing a majority of the material so that the sheet becomes a framework of integrated support members having a small surface area relative to the initial surface area of the sheet of material.. The method also includes positioning the framework around a cylindrical mandrel so that the framework defines at least a partially cylindrical surface or tubular shape. The removing step also includes removing isolated portions of the sheet so that the framework includes a plurality of closed cells bounded by the integrated support members. The removing step is also carried out so that the framework has a fixed length despite a reduction or expansion of the radius of the cylindrical surface or tubular shape. The cylindrical surface or tubular shape has a longitudinal axis and a substantially circular cross-section. The removing step is carried out so that the cylindrical surface or tubular shape is sufficiently flexible about the longitudinal axis to adapt the stent to curved passages within a body vessel without significantly altering the circular cross-section.
The stent of the present invention may also be characterized as a sheet of malleable material which has had a portion of the material removed so that the sheet becomes a framework of integrated support members arranged around a longitudinal axis to define a cylindrical surface. The cylindrical surface has a reduced diameter for delivery of the stent into a passage within a body vessel. The cylindrical surface is also plastically expandable from the reduced diameter to an expanded diameter for holding the passage open. The cylindrical surface has a range of diameters between the reduced diameter and the expanded diameter that are free from overlapping material. Each of the support members of the stent has a width and a thickness significantly less than the width. The support members are integrated in a way that the framework maintains a fixed length when the cylindrical surface is expanded from the reduced to the expanded diameter. One of the support members is a reinforcing member that extends from a first to a second end of the stent. The remaining support members extend laterally on each side of the reinforcing member. The cylindrical surface of the stent also defines a cylindrical surface when expanded to the expanded diameter. In addition, the cylindrical surface is sufficiently flexible about the longitudinal axis so that the stent can advantageously adapt to curved passages within a body vessel without significantly altering its circular cross section. The framework of the stent also includes a plurality of closed cells bounded on all sides by the integrated support members.